The immune system plays a key role in regulating the development and progression of colorectal cancer (CRC), and tumor infiltration by T lymphocytes is a highly informative prognostic factor for CRC outcome. Advances in screening and treatment have substantially improved survival from CRC over the past decade, but clinical outcomes still vary widely among patients with tumors diagnosed at the same TNM stage, and disease relapse occurs in 20-30% of patients with localized cancer. This suggests that traditional prognostic factors, including stage and tumor infiltrating lymphocytes (TIL) quantified by classical means of expert pathology review under light microscopy, lack sufficient granularity to predict patient outcome and to guide the optimal treatment choice for many patients. TILs can trigger preferential lysis of cancer cells by recognizing enhanced expression of abnormally expressed antigens presented in the context of HLA molecules, but the host and tumor factors regulating this adaptive immune response in the tumor microenvironment remain largely unexplored. We propose to address these gaps in knowledge by studying the factors regulating the immune response in an existing, population- based epidemiologic study of 4,000 colorectal cancer cases with essentially perfect clinical annotation and follow-up. A new technical advance now permits the direct sequencing and quantification of the T cell receptor (TCR) repertoire that arises within CRC, providing an opportunity to gain fundamental mechanistic insight within an epidemiologic context. Our goal is to understand why adaptive immune responses differ so dramatically across colorectal cancers, and to harness this information to improve patient outcomes. Here, we hypothesize that clinical, epidemiologic and genetic factors regulate the strength and the quality of the host immune response, and that these factors contribute independently to prognosis. Further, we hypothesize that TIL quantity and TCR clonality in CRC tumors are independent prognostic factors. Combining an innovative high-throughput sequencing approach for TIL characterization (immunoSEQ), a genome-wide association study (GWAS) of genetic variation influencing lymphocyte infiltration in the TME, and a classical epidemiologic investigation of prognosis, we will comprehensively describe the epidemiology of the hostimmune response in tumors from 4,000 patients within a population-based case-control study of CRC in northern Israel. We will identify factors that regulate the host immune response to CRC on a population scale, and advance the current understanding of prognostic factors for CRC, through 3 specific aims. In Aim 1, we will quantify and characterize T cell infiltration in 4,000 primary colorectal cancers (CRCs) from the Molecular Epidemiology of Colorectal Cancer (MECC) study using traditional light microscopy and immunoSEQ. In Aim 2, we will investigate the association of classic epidemiologic variables, HLA locus-specific genetic variation, and genome-wide germline variation with quantitative metrics of the host immune response in 4,000 primary CRCs. In Aim 3, we will evaluate TIL quantity, clonality, and any newly-identified genetic/HLA alleles predictive of immune response as prognostic factors for overall and disease-free survival from CRC. Combining epidemiologic and genetic variables with quantitative and qualitative metrics of the host immune response in the tumor may reveal novel molecular and cellular signatures associated with immune-mediated, tissue-specific destruction.